Hydroxylation of Aromatics with Hydrogen Peroxide over Titanosilicates with MOR and MFI Structures: Effect of Ti Peroxo Species on the Diffusion and Hydroxylation Activity

The shape selectivity in the liquid-phase hydroxylation of aromatic hydrocarbons with hydrogen peroxide has been studied on two kinds of titanosilicate, the large-pore Ti-M with MOR structure and medium-pore TS-1 with MFI structure, and the liquid-phase diffusion of aromatics into both catalysts was compared in the presence of H 2 O and H 2 O 2 to clarify the origin of the shape selectivity. The hydroxylation rate on TS-1 decreased monotonically with increasing molecular size in the order benzene > toluene ≫ ethylbenzene > cumene, while Ti−M showed the maximum rate for toluene hydroxylation. The apparent diffusivity of aromatics was lowered roughly by 1 order by adding H 2 O 2 to H 2 O solvent for both titanosilicates but was not affected for their Ti-free derivatives, mordenite and silicalite-1. The reduction in diffusion rate in the presence of H 2 O 2 was more pronounced for TS-1 and for Ti-rich samples. It is concluded that a bulky Ti peroxo species (Ti−OOH) formed by the interaction of Ti site with H 2 O 2 mainly causes a transition-state shape selectivity in the hydroxylation of bulky aromatics in titanosilicate/H 2 O 2 systems.